1. Technical Field
The present invention relates to a charge controller for rechargeable, secondary batteries, and more particularly, to a charge controller that can effectively manage electricity from a power source of limited current capacity to charge a secondary battery while supplying power to load circuitry.
2. Discussion of the Background
Rechargeable, secondary batteries are commonly employed in portable electronic devices, such as laptop computers and cellular phones. Typically, battery-powered electronic equipment includes charge control circuitry that simultaneously charges the secondary battery and supplies power to load circuitry whenever the device is connected to an external power source, for example, an AC adapter. A problem encountered by the charge controller is that it fails to supply the secondary battery with a sufficient current when the electronic device is in use and consumes power, where the external power source has a limited maximum current output capacity equivalent to, or slightly greater than, a maximum rated current of the load circuitry.
To address this problem, various techniques have been proposed to provide an effective charge control circuit that can adjust a charge current depending on a load current being supplied to the load circuit.
For example, one conventional method provides a charge control circuit that draws power from a current-limited, constant voltage AC adapter for supplying a charge current and a load current, which varies with the operating state of load circuitry. According to this method, the charge control circuit can maximize the charge current to an extent that does not cause the output voltage of the AC adapter to fall below a minimum allowable limit. In particular, the charge control circuit can maintain the sum of the charge current and the load current within a range with which the AC adapter can supply power at a constant voltage.
Another conventional method provides a charge control circuit that draws power from a current-limited power source. According to this method, the control circuit can adjust a charge current based on a difference between a measured load current and a programmed charge current, so as to maintain the sum of the load current and the charge current within a maximum current limit for the current-limited power source.
These conventional methods have several drawbacks due to the operating principle that the sum of the load current and the charge current always matches the maximum current limit of the AC adapter.
One common drawback is that the charge control circuit has substantially no tolerance for excessive current where the current rating of the AC adapter is relatively small. That is, a significant decline in the output voltage of the AC adapter can occur to cause malfunctioning of the device, when the load current increases so rapidly that the circuit fails to cancel the increase in the load current by reducing the charge current.
Another common drawback is that the charge current can become significantly large to exceed a rated maximum limit of the secondary battery where the current rating of the AC adapter itself is relatively large.
In addition to those drawbacks that may result when the secondary battery is in a normal operating state, still another drawback can also occur where the battery is in an abnormal state, e.g., over-discharged or overheated. An over-discharged or overheated secondary battery is required to be charged with a current sufficiently smaller than normal, since charging with a large current can reduce the life of the battery in the abnormal condition. However, the conventional circuit cannot adjust the charge current depending on the operating condition of the battery, and thus has the risk of damaging the battery during charge.